GCD的Queue-Specific

为了能够判断当前queue是否是之前创建的queue，

我们可以利用dispatch_queue_set_specific和dispatch_get_specific给queue关联一个context data，

后面再利用这个标识获取到context data。

如果可以获取到说明当前上下文是在自己创建的queue中，如果不能获取到context data则表示当前是在其他队列上。

 　　 NSString *queueKey = @"queueKey1";
     dispatch_queue_t queue = dispatch_queue_create("testQueue", DISPATCH_QUEUE_CONCURRENT);
     dispatch_queue_set_specific(queue, (__bridge const void * _Nonnull)(queueKey)
                                 , &queueKey, NULL);

     NSLog(@"1.当前线程:%@ 队列:%@", [NSThread currentThread], dispatch_get_current_queue());

     if (dispatch_get_specific((__bridge const void * _Nonnull)(queueKey))) {
         NSLog(@"2.当前线程:%@ 队列:%@", [NSThread currentThread], dispatch_get_current_queue());
     }else{
         NSLog(@"3.当前线程:%@ 队列:%@", [NSThread currentThread], dispatch_get_current_queue());
     }

     dispatch_sync(queue, ^{

         if (dispatch_get_specific((__bridge const void * _Nonnull)(queueKey))) {
             NSLog(@"4.当前线程:%@ 队列:%@", [NSThread currentThread], dispatch_get_current_queue());
         }else{
             NSLog(@"5.当前线程:%@ 队列:%@", [NSThread currentThread], dispatch_get_current_queue());
         }
     });

输出信息

2018-02-13 16:44:13.977719+0800 CGD[14596:276910] 1.当前线程:<NSThread: 0x604000260180>{number = 1, name = main} 队列:<OS_dispatch_queue_main: com.apple.main-thread>

2018-02-13 16:44:13.977900+0800 CGD[14596:276910] 3.当前线程:<NSThread: 0x604000260180>{number = 1, name = main} 队列:<OS_dispatch_queue_main: com.apple.main-thread>

2018-02-13 16:44:13.978047+0800 CGD[14596:276910] 4.当前线程:<NSThread: 0x604000260180>{number = 1, name = main} 队列:<OS_dispatch_queue: testQueue>